Rejuvenator for grinding disc



March 22, 1966 D. OLSON REJUVENATOR FOR GRINDING DISC 2 Sheets-Sheet 1 Filed Feb. 24, 1964 INVENTOR. 04 W0 OLSON BY W All: fi'rrogms'ys.

March 22, 1966 Filed Feb. 24, 1964 D1 OLSON REJUVENATOR FOR GRINDING DISC 2 Sheets-Sheet 2 I I I INVENTOR. ,DH v/p (Dz. so/v Big/W n! M: flrromve Y United States Patent 3,241,268 REJUVENATOR FOR GRINDING DISC David Olson, 10240 Regan St., San Jose, Calif. Filed Feb. 24, 1964, Ser. No. 346,727 Claims. (Cl. 51358) This invention relates to abrasive disc grinding and more particularly to a grinding disc rejuvenator.

Grinding discs are used extensively in vehicle body shops for resurfacing body panels, fenders and bumpers prior to painting and/or plating of such parts. In such grinding a hand type portable grinding tool is employed. Such tools consist of a rotary shaft having backing plates mounted thereon as a base for the grinding disc which is a replaceable heavy fabric disc having a coating of grit of a desired coarseness adhering thereto for abrading a surface to be ground.

Such grinding discs in time lose their effectiveness and cease to cut adequately under normal circumstances. As a rule this is not because the grit wears out but because of foreign matter becoming lodged between the grit. Such foreign matter is created by loosened paint and particles of metal becoming gummy under the heat of the grinding operation and due to the pressure of the grinding disc against the surface being abraded. The foreign matter becomes gummy under the heat of the grinding operation and in such gummy condition is pressed into the spaces between the grit thus plugging the same. Consequently only the grit which extends beyond the compacted foreign matter affords cutting action. As grinding continues the extended portions of the grit wear down and further film of foreign matter builds up until a relatively smooth surface is created and abrading action drops to a minimum.

There have been many attempts at reconditioning used grinding discs. Solvents and cleaners have been tried for loosening the foreign matter but much of the latter is so compressed, solidified and mixed with non-soluble substances as to be impervious to such solvents. Wire brushes have also been used with little or no success.

The present invention is directed to a device for mounting used grinding discs in such a manner as to flex the disc during use to thereby open up the spaces between the grit and obtain further abrading action.

It is an object of this invention to provide a grinding disc rejuvenator for obtaining a flexing action at radial zones of a used disc. In this connection it is a further object to provide such a rejuvenator by which different radial zones can be presented at various stages of wear and use of such used grinding discs.

It is another object of this invention to provide a rejuvenator having radial lands each presenting a surface of coarse grit adapted to bear against the back face of a used grinding disc. The grinding disc is thus pressed outwardly to thereby open up the spaces between the grit on the used disc. Along with this object it is further contemplated that the coarse grit surface on the radial arms of the rejuvenator embed themselves into the back face of the used disc to resist the tendency of slippage between the latter and the rejuvenator arms.

These and other objects and advantages of the present invention will become apparent from a reading of the following description and claims in the light of the accompanying two sheets of drawings in which FIG. 1 is a perspective view of a portable grinder having the grinding disc rejuvenator of the present invention embodied therein.

FIG. 2 is a fragmentary side elevation of a portion of FIG. 1 and at slightly larger scale.

FIG. 3 is an exploded perspective view of the grinder shaft, grinding disc and mounting therefor with the rejuvenator of the present invention interposed therein.

FIG. 4 is a modified form of the rejuvenator illustrated in FIG. 3.

FIG. 5 is another form of the rejuvenator of FIG. 3.

FIG. 6 is an enlarged section through one arm of the rejuvenator juxtaposed between a grinding disc and a backing plate therefor.

Referring to the drawing the general arrangement of a conventional grinder will first be described.

A portable grinder is shown at 10 as a hand type tool by which a grinder man can abrade surfaces of various shapes and contours such as are found on auto bodies, bumpers and the like. Such grinder 10 usually includes a housing 11 for a self-contained motor with power reduction gears (not shown) for turning a drive shaft 12. The drive shaft 12 extends from a bearing boss 13 formed as a part of the housing 11 of the grinder 10. Beyond the bearing boss 13 the drive shaft terminates in a reduced threaded end 14 for receiving the grinding disc and backing plates therefor.

A cone shaped base plate 15 is first mounted on the threaded end 14 of the drive shaft 12. This base plate 15 rests against a shoulder 16 between the bearing boss 13 and the threaded end 14 of the shaft 12. The coneshaped base plate 15 has an outer periphery 17 which is disposed about half the distance radially from the center as compared with a conventional sanding disc which is usually about 9 inches in diameter.

In addition to the cone shaped base plate 15 several step up backing plates are mounted on the shaft 12. These consist of a first backing plate 18 of about 7 inches diameter and second backing plate 19 of 9 inches diameter. Both plates 18 and 19 are flat discs which are relatively flexible to give a reasonable degree of firmness to the sanding disc 20 mounted on the outer face of the last backing plate 19.

A washer 21 and a nut 22 are finally mounted on the extreme end of the threaded shaft 14 to secure the base 15, the several backing plates 18 and 19 as well as the disc 26 firmly on the shaft 12 for turning therewith.

When the grit of the grinding disc 20 becomes plugged up with foreign matter and ceases to cut etficiently, the disc 20 is removed. In accordance with the present invention a rejuvenator 25 forming the subject of this application is inserted between the outer backing plate 19 and the disc 20. The Washer 21 and nut 22 again are placed upon the threaded end 14 of shaft 12 to secure the grinding disc and plates to the grinder shaft 12.

The rejuvenator 25 comprises a fiat layer of backing material 26 similar to that employed in a grinding disc such as the one 20. A layer of grit 27 on this backing material 26 adheres to the latter by suitable cement. The material 26 has a pluraltiy of radial arms 28, for example, six or eight as illustrated in FIGS. 3, 4 and 5 of the drawing. These arms 28 are relatively fiat and preferably of a width of about one inch across. The arms 28 extend radially from a central zone 29 having an axial bore 30 adapted to fit the threaded end 14 of the shaft 12. In effect the rejuvenator 25 might be said to be a disc comparable in size to a grinding disc and having an abrasive surface 31 from which pie shaped segments are cut leaving voids 32 between a plurality of relatively flat radially extending lands 28 about a central zone 29.

The grinding disc rejuvenator 25 when placed between the final backing plate 19 and a used grinding disc 20 serves to support the latter in the same relatively flat condition as before. However, due to the voids 32 between the lands 28 the portions of the disc 20 overlying such voids tend to be flexed inwardly. This is so especially due to the force with which the grinding dis-c 20 is pressed 3 against the surface to be abraded. The phenomenon is as follows:

Assuming the grinder shaft 12 to be turning counterclockwise as indicated by the arrow a in FIG. 4, the leading edge 35 of each land 28 provides a comparable radially extending cutting zone 36 with a straight leading edge 37 on the used grinding disc-2t). The unsupported area of the disc ahead of this straight leading edge 37 sinks in sufficiently to present a straight cutting edge 37 of protruding grit. The action is very much like a straight cutting edge of a rotary blade. The grit on the abrasive surface 31 of each land 28 becomes embedded into the back surface of the used grinding disc 20. This has two important advantages. First it maintains the rejuvenator in relatively static or fixed relation to the grinding disc 20 and; secondly, the protruding facets of the grit on the abrasive surface 31 tend to bow the disc 21] and open up the plugged up spaces between the grit on the used grinding disc 20.

In connection with these two advantageous features, as explained first above, I have found the use of coarser grit on the rejuvenator to be essential to my invention. In other words, by using on the rejuvenator 25 grit that is from one and a half to slightly greater than twice the size of that on the used grinding disc best results are obtained. For example, if the grit on the disc 20 is grains to the square inch the grit on the rejuvenator should be from 24 to 36 grains to the square inch. If the grit on the disc 20 is 36 grains to the square inch 2. range of from 16 to 24 grains to the square inch on the rejuvenator is preferred.

By this differential the coarser grit in each instance digs into the back of the fabric sheet on which the grit of the grinding disc 20 is mounted. More important, however, the larger grit on the rejuvenator lands serves to push up the underside of the grinding disc to thereby bow the same and to open up the spaces between the smaller grit on the grinding disc.

As a result of these effects on the used disc the several radial cutting zones afford greater abrading action than could be obtained from the used disc without the rejuvenator beneath it. The straight cutting zones 36 wear down somewhat as illustrated in FIG. 4 wherein the horizontal shading indicates that the grit has been worn away to expose the fiat smooth surface of the heavy fabric base of the disc 20 upon which the grit had been cemented. From this observation it appears that foreign matter, grit and gum removed during grinding with a used disc backed by the rejuvenator 25 is cast back into the depressed zones of the disc which overlie the voids 32 on the rejuvenator. Consequently the normal effect of having the foreign matter build up and plug the grit is avoided during use of the rejuvenator 25. Therefore, full use of the cutting effect of the grit results at least in the cutting zones 36 which overlie the fiat lands 28 of the rejuvenator.

When the cutting zones 36 become smooth in the mannar as illustrated in FIG. 4, the nut 22 is again loosened. The used disc 20 is then shifted segmentally, for example, clockwise relative to the rejuvenator to dispose the lands 28 thereof beneath the non-worn areas of the disc 20. The nut 22 is again tightened and the used disc is again ready for use. This time the non-worn zones which had previously overlaid the voids 32 are now backed by the lands 28. The rejuvenated cutting action is again as explained above until the used disc is almost entirely smooth. By this arrangement about twice as much effective and etficient grinding time is obtained from a single disc. Experience has proven that where 8 to 10 discs per day were required, only 3 to 6 discs are now required when the rejuvenator of the present invention is employed, and this occurs without treating or otherwise reconditioning a gummed up used disc. When the disc ceases to cut or grind effectively it is merely removed and the rejuvenator 25 placed between it and the outerbacking plate 19. The flat lands 28 and coarser grit thereon affords the action as previously explained to obtain the maximum use of the grinding disc.

While I have explained the rejuvenator for grinding discs in specific detail it will be appreciated that it may be susceptible to variations, modifications and/ or alterations without departing from the spirit of my invention. I therefore desire to avail myself of all variations, modifications and/ or alterations as may fairly come within the purview of the appended claims.

What I claim as new and desire to protect by Letters Patent is:

1. In combination with a grinding disc mounted on the backing plate of a rotary grinder shaft, means for rejuvenating such a grinding disc after it has become plugged up with foreign matter between its grit, said means comprising a layer of coarser grit than that on such used grinding disc arranged as a plurality of fiat radially extending lands between the grinding disc and backing plate.

2. In combination with a grinding disc mounted on the backing plate of a rotary grinder shaft, means for rejuvenating such a grinding disc after it has become plugged up with foreign matter between its grit, said means comprising a layer of coarser grit than that on such used grinding disc arranged as a plurality of fiat radially extending lands between the grinding disc and backing plate, whereby to press said grinding disc outwardly to present straight cutting edges above the leading edges of said lands and inwardly flexed zones between said lands wherein foreign matter and grinding dust slough off without becoming embedded into and attached to the used grinding disc.

3. In combination with a grinding disc mounted on a backing plate for turning therewith, a rejuvenator for gummed up used grinding discs comprising a disc of comparable material mounted between such used grinding disc and the backing plate having a plurality of flat radial lands of grit thereon of greater size than that on the used disc.

4. In combination with a grinding disc mounted on a backing plate for turning therewith, a rejuvenator for gummed up used grinding discs comprising a disc of comparable material mounted between such used grinding disc and the backing plate having a plurality of fiat radial lands of grit thereon of greater size than that on the used disc for bighting into the back face of the used disc to maintain the latter in fixed static relation to said lands and for bowing the used disc outwardly to open up the plugged up grit thereon.

5. In combination with a grinding disc mounted on a backing plate of a rotary grinder, a rejuvenator for gummed up used grinding discs comprising a disc of comparable material mounted between such used grinding disc and the backing plate having a plurality of fiat radial lands of grit of from between one and one half to double the size of the grit on the used disc, whereby to press said grinding disc outwardly to present straight cutting edges above the leading edges of said lands wherein foreign matter and grinding dust slough off without becoming embedded into such used grinding disc.

No references cited.

LESTER M. SWINGLE, Primary Examiner.

LEONARD S. SELMAN, Assistant Examiner. 

1. IN COMBINATION WITH A GRINDING DISC MOUNTED ON THE BACKING PLATE OF A ROTARY GRINDER SHAFT, MEANS FOR REJUVENATING SUCH A GRINDING DISC AFTER IT HAS BECOME PLUGGED UP WITH FOREIGN MATTER BETWEEN ITS GRIT, SAID MEANS COMPRISING A LAYER OF COARSER GRIT THAN THAT ON SUCH USED GRINDING DISC ARRANGED AS A PLURALITY OF FLAT RADIALLY EXTENDING LANDS BETWEEN THE GRINDING DISC AND BACKING PLATE. 